As a conventional valve device, a valve device that includes a housing that has a fluid passage, a valve shaft that is held by the housing in a rotatable manner, a butterfly valve with a circular plate shape that is secured to the valve shaft and opens and closes the fluid passage, and two semicircular-arc-shaped sheet parts that are formed to protrude from an inner surface of the fluid passage such that the butterfly valve is brought into contact with the semicircular-arc-shaped sheet parts dining closure, and the like is known (see Patent Literature 1, for example).
In this valve device, contact surfaces of the sheet parts with which the butterfly valve is brought into contact are disposed at positions that overlap with bearing holes of the valve shaft in an extending direction of the fluid passage. Therefore, it is not possible to provide the sheet part in ranges in which the valve shaft and the butterfly valve rotate.
In addition, although a covering part that is continuous with the sheet parts is provided to partially cover the circumference of the valve shaft, it is not possible to cover the entire circumference of the valve shaft.
Therefore, there is a concern that fluid leakage may occur from the upstream side toward the downstream side in the region where the sheet parts and the covering part are not present.
In addition, another valve device that includes a housing that has a fluid passage, a valve shaft that is held by the housing in a rotatable manner, a circular plate-shaped butterfly valve that is secured to the valve shaft via a spacer to open and close the fluid passage, two arc-shaped stepped surfaces that are formed in an inner circumferential surface of the fluid passage with which the butterfly valve is brought into contact during closure, and the like is known (see Patent Literature 2, for example).
In this valve device, the stepped surface with which the butterfly valve is brought into contact is disposed at a position set apart from a bearing hole of the valve shaft in an extending direction of the fluid passage, and the butterfly valve is secured to the valve shaft at a position deviating from the rotation center of the valve shaft by a prescribed amount.
Therefore, in order for the butterfly valve to be able to rotate to a closing position, it is not possible to provide the stepped surface such that an outer edge (diameter part) of the butterfly valve does not interfere with the stepped surface in a region in the vicinity of the valve shaft in which the butterfly valve is turned.
Therefore, there is a concern that fluid leakage may occur from the upstream side toward the downstream side in the region where the stepped surface is not present.